Fix Content-MD5 header for s3transfer

[nateprewitt]

Overview

This is a continuation of #1985 to resolve #1979. We're adding two additional tests to prevent future regression with empty bodies, and adding some more info on the underlying issue.

Bug

While the initial PR was addressing a lack of Content-MD5 headers being added for empty files going to s3, the bug is little more subtle. Currently, using the low level API for PutBucket will ensure all input, regardless of length, is transformed into a file-like object. When this is passed to our MD5 checksum function, it will always be True and get the header added correctly. These always evaluate to True because bool resolves "truthiness" to be if the file-object exists or not.

The bug enters when the higher level s3 APIs are being invoked like s3 sync in the AWS CLI. This replaces our standard file-like wrappers (StringIO, BytesIO) with s3transfer's ReadFileChunk. Due to s3transfer implementing __len__ on what would otherwise appear to be a file-like object, we now get different criteria for what makes it truthy. bool will choose __len__ > 0 as being more specific than if the file exists. That broke our previous assumptions on what would be coming through to conditionally_calculate_md5.

Outcome

We probably should make ReadFileChunk adhere closer to the file-like object spec, but at this point it would be a backwards incompatible change. I think the original proposal from @wimglenn is the safest bet to make things operate as expected.

[codecov-io]

Codecov Report

Merging #2221 (b263f6e) into develop (60d2bf7) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2221      +/-   ##
===========================================
- Coverage    98.23%   98.23%   -0.01%     
===========================================
  Files           58       58              
  Lines        10852    10848       -4     
===========================================
- Hits         10661    10657       -4     
  Misses         191      191              

Impacted Files	Coverage Δ
botocore/handlers.py	96.35% <ø> (-0.04%)	⬇️
botocore/utils.py	97.73% <100.00%> (ø)

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[zdutta]

Looks good!